Cathode-ray tube



June L. A. -WOODBRIDGE..V 2,472,766

cATHopE-RAY TUBE Filed Dad. 21, 1945 I/VVENTOR Leonard A. Woodbr'jye Patented June 7, 1949 CATHODE-RAY- TUBE Leonard Albert WoodbridgaqWoodford, England, assignor toA. C. Cossor Limited,r.London,lEng land, a British company ApplicationD'ecember 21, 1945, Serial No. 636,454

U'Nl-TE'D STATES; o-FF-IcE In Great Britain Decembenas28g1944-i Claims.

difierence inresponsiveness of electrons and negative ions tothe defiectingforce of a; magnetic field. Thus in" somev arrangements; the electrodes inv the-"neighbourhood: oi the cathode 1 are. so :arranged as .-to--produce. a; divergent beam of :electronsthaving an-raxis inclined :to :the. axis of the Convergent beam in which;v the electrons will finally; enterthe: region of variable. deflection fields. Thebeam, preferably before it reaches therintermediate region; in-which :it is converted from divergence to convergence. is passed through: a-steadymagnetic field whereby the electrons .aredefiected'until their axis coincides with theraxis ofithe .path' along which :they will finally enter the region of the variable adeflection fields. The negative ions, which are undesired, but are present inthe sdivergentb'eam.entering.the steady magnetic field, are not appreciably deflected by this field. They; therefore; strike either an electrode or. the .side. of thetube-envelope, and are prevented from reaching the target.v Preferably they are so directed as to strike a conducting coating on'the side of the neck of the'tube envelope or an electrode situated' within this neck.

It is an object ofrthe invention, while-.;providing means for preventing ions from reaching the target, to permit the normaldesign to be retained in which the envelope andxthe gun electrodes are symmetrical about a common axis,

thisbeinga very great convenience in manufacture. The whole..envelope..oi.the tube is rotationally symmetrical about the axis of its neck; and the necessary inclination, with respect to that axis, ,otthe beam. which issues from the preliminary triode, or equivalent structure, is obtained by asymmetrical placing of the electrodes thereof.

It is a further object of the invention to permit the construction of an electron gun which whilst having a symmetrical arrangement of its electrode discs and other parts about a common axis substantially separates electrons and negative ions in the beam, trapping the ions, and yet provide a resultant electron beam which is paral- ""metrical aboututheaxis Il=of'the-tube is well H.. Insorder to counter-actth-isskewing, a perlel .-to .or coincident with the saidv common axis, this object beingachieved by-the eccentric positioning of .the electrode apertures with respect to..the saidaxisandby the application of a single. transverse magnetic field.

TheHfollowing, description relates. to the accompanying drawing which shows a diametral sectionthroughthe: neck of a-cathode ray tube embodying the invention.

Within .theneckl of the cathode ray tube is located an.electrode=supporting frame consisting ofparallelrods 2...andspacedinsulating discs 3. Of the .three .discs, shown, the lowermost supports thecathode .4 and grid 50f the electron gun while the-middle'and uppersupport thefirst anode 6.

'The..cathode 4 isof fiatttype, with the heating element 1 arranged internally thereof. The grid 5 is in the form of an apertured disc which is dishedutowardsthe cathode. The'first anode 6 is, in the formof a.closed.cylinder with aperturesinitsendwalls: thusthe lower end wall 8 of the cylinderis the. anode proper, the peripheral portions .constitutes a. shield, and the upper end wall I! 0. .is .-the anode. diaphragm.

The. assembly, as so far-described; being symknown. Inaccordance'with thepresent invention, however, the. aperture l2= inthe grid 5-and the aperture l 3.-in-. the anode 8* are each ofiset from the-.,.axis. ll--H in-thesameradial direc- This. offsetting. causes. the-v electron beam emerging, from. the; gird aperture l2 to be skewed with respect to or to cross the-desired path which is parallel .or..pref.erablycoincident-with the axis manent. or electromagnet having a pole M as indicated. in the. dottedlines in the-figure is'arranged to. develop in the interior of the cylindrical. anode ,6..a magneticfield of such direction andjstrength. astotcausel the electron stream to be restoredto parallelism with the axis II. The path rfollowed by the. axis 0fthe .electronbeam is.indicated-.generally byzthedashed line B in the drawing, and it will be seen that this line coincides with the axis II as it passes through the aperture M in the diaphragm II). On the other hand, negative ions associated with the stream, not being so substantially affected by the magnetic field, fail to find their way through aperture l4 and tend to be collected on the inner wall of the cylinder, whereby the screen of the oathode ray tube is protected from their effect.

The ofi'setting of the grid aperture l2 in the same direction as the anode aperture l3 should be such that the apertures overlay in order to permit a beam of electrons to enter cylinder 6. The electron beam, after being skewed by the ofiset aperture 13 and restored to parallelism by the magnetic field, is preferably brought not only into parallelism with the axis, but coincident with it, so that it emerges from the aperture [4, which is central in the anode diaphragm I0, exactly as it would in the case of a normally-constructed cathode ray tube.

Thus when the grid and first anode apertures are offset in the same direction to the appropriate extents and a restoring magnetic field is ap: plied of an appropriate strength, then the fluorescent screen is not so liable to burning as hitherto.

It may be noted as one desirable feature of the construction illustrated that the insulating disc 3 which supports the cathode 4 lies substantially in the same plane as the cathode. In some previous constructions the plane of the supporting disc is below that of the cathode; tilting of the disc out of a plane normal to the axis ll-l l is not liable to afiect the amount of offset of the grid aperture appreciably in relation to the cathode or the position of either relatively to the axis.

In an actual embodiment of the invention the dimensions were as follows:

The grid aperture l2 had a diameter .8 mm. and its centre was offset from the axis H by a distance of 1 mm. The anode aperture 13 had a diameter 4 mm. and its centre was ofiset by 2.5 mm. The distance between the two apertures in a direction parallel with the axis II was 3.5 mm. The dished portion of grid 5 was 2 mm. in depth.

I claim:

1. A cathode ray tube comprising an evacuated envelope having a neck-portion and a bulbous portion symmetrical about a common axis with a fluorescent screen On the end of the said bulbous portion, an electron gun arranged within said neck portion symmetrically about said axis but having its grid and first anode apertured eccentrically with respect thereto in the same radial direction so that an emergent electron and ion beam is skewed with respect to said axis, and deflecting means acting upon said emergent electron and ion beam to restore the electron portion of the beam to coincidence with the said axis but leaving the ionic portion of the emergent beam relatively undeflected so that the said ionic portion strikes a part of the cathode ray tube other than the said fluorescent screen.

2. A cathode ray tube comprising an evacuated envelope having a neck portion and a bulbous portion symmetrical about a common axis with a fluorescent screen on the end of the said bulbous portion, an electron gun arranged within said neck portion symmetrically about said axis in which the first anode is apertured eccentrically with respect to said axis so that an emergent electron and ion beam is skewed so as to cross said axis, and deflecting means acting upon said emergent electron and ion beam to restore the electron portion of the beam to parallelism with the said axis but leaving the ionic portion of the emergent beam relatively undefieeted so that the said ionic portion strikes a part of the cathode ray tube other than on the said fluorescent screen.

3. A cathode ray tube as claimed in claim 1 in which the electron beam projected from the gun is directed towards a conducting surface within the envelope separate from the said fluorescent screen.

4. An electron gun for cathode ray tubes comprising an anode formed of a closed metallic cylinder, a cathode mounted adjacent one end of said cylinder and on the axis thereof, said end of said cylinder having an aperture formed eccentric to said axis, whereby an electron and ion beam enters said anode cylinder in a direction tending to cross said axis, the other end of said cylinder having a central aperture formed at the axis of the cylinder, and deflecting means acting upon said electron and ion beam within said anode cylinder to deflect the electron portion of said beam and to direct said electron portion out of said central aperture in coincidence with the axis of said anode cylinder. 5. An electron gun for cathode ray tubes comprising, an anode formed of a closed metallic cylinder, a cathode arranged adjacent one end of said cylinder and on the axis thereof, a disk-like grid interposed between said cathode and the adjacent end of said anode, said grid and the adjacent end of said anode having apertures formed therein with their axes eccentric with respect to the axis of said anode and in the same radial direction whereby an electron and ion beam is directed into said anode from said cathode in a direction at an angle to said axis, and deflecting means acting on said beam within said anode for deflecting the electron portion of said beam and directing said portion out of said anode through a central aperture formed in the other end thereof and along a path coincident with the axis of said anode.

LEONARD ALBERT WOODBRIDGE.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS 

